The present application relates to biomedical devices, such as dental crowns, bridges and implants, and other devices that can be employed within people or animals.
The manufacture of dental devices such as crowns has traditionally been a labor-intensive process. After a dentist has ground away parts of a patient's tooth to leave a protruding tooth stub, an impression of the stub is taken, typically along with an impression of the surrounding and opposing teeth. The impression is typically sent to a dental laboratory, which makes a mold from the impression of the stub, the mold commonly made of gypsum and called a die stone. For crowns that are made completely of metal such as gold, silver, palladium or stainless steel, a wax replica of the tooth is then hand crafted on the die stone, and the wax tooth replica is then invested in a material that solidifies. As that material is heated the wax is melted or burned out. The molten metal is then poured into a void created by the lost wax, and cools to become the crown.
Conventional fabrication of porcelain crowns is complicated by attempting to match the crown to the color of the surrounding teeth. In this case, the crown is usually formed atop a metal coping that may impart a color that is apparent through translucent porcelain. For instance, a titanium (Ti) coping readily oxidizes to have a titanium oxide (TiO or TiO2) film, which is black and can affect the color of the porcelain crown.
Such a metal coping is usually made by the lost wax technique to fit atop the tooth stub. Lost wax casting of titanium requires specialized vacuum casting equipment not typically used for noble and base metal alloys, to avoid denegrating the titanium. On the other hand, metal copings made of noble metals such as gold (Au), silver (Ag) and/or palladium (Pd) or base metals such as nickel (Ni) or beryllium (Be) can cause allergic reactions. After casting, the cast metal coping is hand finished to a thickness as small as 0.2 millimeters (mm). Such hand working is difficult and can result in holes in the coping that require the coping to be recast.
Prior to applying porcelain to the coping, the mandrel is removed. Titanium copings usually have a covering of opaque porcelain baked on the coping to hide the TiO/TiO2 film. The baked-on porcelain also helps to bond the porcelain veneer to the titanium coping. The translucent porcelain layers are then applied atop the baked porcelain layer and fired. After the porcelain has been fired and machined to finish the crown, the metal and porcelain crown are bonded to the tooth stub.
Recently, instead of forming a crown or coping with the lost wax technique, a computer aided design/computer aided engineering (CAD/CAM) process has been developed to shape copings for porcelain crowns. The machinery for this is fairly expensive, however, and the coloring of the coping can still affect the crown color. Moreover, pressure from the automated tools that shape the coping becomes problematic at a coping thickness of a few tenths of a millimeter.